When graphite is incorporated into a thermoplastic resin, the thermal conduction properties of the resin are improved according to the amount of the graphite incorporated. In the thermoplastic resin having incorporated thereinto an increased amount of graphite which is not melted in melt-kneading, the proportion of the thermoplastic resin which is melted in melt-kneading is reduced, and therefore it is difficult to maintain the high productivity of the thermoplastic resin in the melt-kneading using a single-screw or twin-screw extruder. Patent document 1 discloses that kneading is performed in a state such that the head portion of an extruder is opened. However, this patent document has no disclosure of a cooling apparatus, such as a water bath, for efficiently removing heat from the resultant pellets in a flake form, and there is a fear that the pellets stick together, and such kneading is not preferred from the viewpoint of the molding processability.
The thermoplastic resin having graphite solely incorporated thereinto exhibits unsatisfactory physical properties, such as strength. Patent document 2 has a description showing that, by incorporating into a thermoplastic resin specific amounts of graphite and carbon fibers having a thermal conductivity of 100 W/mK or more, the resin is improved in flexural strength and thermal conduction properties. However, there is no disclosure of PAN carbon fibers generally used, which have a thermal conductivity of about 10 W/mK and which are obtained by carbonizing polyacrylonitrile fibers.
Polyamide resins, such as polyamide 6 and polyamide 66, have excellent properties and can be easily melt-molded, and therefore are widely used as general-purpose engineering plastics. Reference documents 3 and 4 disclose that, by incorporating magnesium oxide into the polyamide resin, the resin is improved in thermal conduction properties.
On the other hand, as the increase in density and the miniaturization of an electronic device progress, the amount of the polyamide resin used per part for electronic device is reduced, and hence the effect of the properties of the polyamide resin used on the performance of the part for electronic device is becoming large. In accordance with this, there are increasing demands of the improvement of the properties of the polyamide resin. Especially, there are increasing demands of prevention of the lowering of electrical insulating properties of the polyamide resin, which strongly affects the performance of the part for electronic device, under high-temperature and high-humidity conditions (after a high-temperature and high-humidity treatment), which are presumed to be, for example, under conditions in the summer in Japan.
When magnesium oxide is incorporated into a thermoplastic resin, the thermal conduction properties of the resin are improved according to the amount of the magnesium oxide incorporated. In the thermoplastic resin having incorporated thereinto an increased amount of magnesium oxide which is not melted in melt-kneading, the proportion of the thermoplastic resin which is melted in melt-kneading is reduced, and therefore it is difficult to maintain the high productivity of the thermoplastic resin in the melt-kneading using a single-screw or twin-screw extruder. As a method of stably filling a resin with an increased amount of a conductive filler, patent document 1 discloses that kneading is performed in a state such that the head portion of an extruder is opened. However, there is no disclosure of a method of stably filling a resin with an increased amount of a conductive filler without opening the head portion of the extruder.
Further, patent document 5 discloses a method for improving the moldability, appearance, and thermal conduction properties by incorporating magnesium oxide having a specific particle size in a specific amount. However, in the resultant molded article, the thermal conductivity varies depending on the location of measurement, and there is no disclosure that a molded article exhibiting a uniform thermal conductivity irrespective of location of measurement is stably obtained.